Electronic digital communication systems are in wide use and find application in a variety of process control systems including those related to building automation systems (BAS) used for heating, ventilating and air conditioning (HVAC) control and related energy management, fire and security applications. In known BAS architectural networks, an exemplary multistory office building may be equipped with a computerized central processing unit (CPU) which is electrically coupled to lower-level electronic controllers and thence to the working apparatus such as mechanical heaters, chillers, fire detection and control hardware, security devices and other equipment. The CPU, the other electronic controllers and the mechanical equipment are frequently coupled together for electronic communications therebetween by shielded coaxial cable using relatively large diameter conductors, 18 ga. being exemplary. Such dedicated wiring permits the transmission of voltage signals having relatively large amplitude to minimize the effect of noise and signal attenuation. The topology of such a network is characterized by a predominance of relatively short interconnections and "daisy chain" links to minimize the total wiring which is required to be installed in the building.
More recently, it has been noted that a typical building voice communication system will include one or more private branch exchanges (PBX) and a large number of voice communication devices such as telephones, all interconnected by relatively long runs of small diameter, unshielded twisted pair wires arranged proximate one another to define wiring bundles. Each wire of a twisted pair may typically be of 24 ga. size, thereby permitting a low cost installation suitable for the conduction of relatively small amplitude voltage signals to avoid undesirable cross interference between pairs. The use of the PBX wiring for effecting BAS communications is highly desirable from an overall installed cost standpoint. However, the aforedescribed differences in network topology and wiring electrical characteristics require a novel approach for best data communications.
One approach to the use of PBX wiring for data communications is to use telephone modems with analog active bridging. With this approach, the active bridge both receives and transmits analog signals of varying frequency which are summed and amplified by the bridge. Another approach is to use digital rather than analog signals by employing one twisted pair for the transmission and reception of control signals and a second twisted pair for the transmission and reception of the related digital data signals. While these approaches have heretofore been satisfactory, they tend to be characterized by certain disadvantages. In particular, analog active bridging has attendant thereto an inordinantly high cost as such equipment is normally designed for voice grade telephone networks. In the latter approach using digital signals, two twisted pair (four individual wires) are required for communication and control. This necessitates the installation of additional PBX wiring for each communication channel and unnecessary costs will result. A method and apparatus for effecting digital communications over a single twisted pair telephone line which uses a plurality of voltage levels to provide both control and digital data signals would be an important advance in the art.